A conventional electrophotographic printing device (image forming device) includes a photoconductive drum (image carrying body), a charging device, an exposing device, a developing device, a transferring device, and a fixing device.
In such a printing device, the photoconductive surface of the photoconductive drum is subjected to charging and exposure so as to form an electrostatic latent image formed thereon, and the formed latent image is then developed by toner (developer), thereby forming a toner image (visible image). Further, the toner image is transferred onto a sheet (recording medium: a printing medium like a general paper, an OHP sheet etc.), followed by thermal fixing.
Further, the developing device of the printing device generally includes a toner containing tank (toner tank) for keeping the toner. The toner tank has a function of charging the contained toner for development through friction charging or charge injection.
Further, a developing roller is provided in the opening section of the toner tank. The developing roller rotates in contact with the photoconductive drum, with the toner adhered thereon, thereby serially (continuously) adhering the toner onto the surface of the photoconductive drum. In the vicinity of the developing roller, a blade (layer thickness controlling member) is provided to flatten the toner layer on the roller surface.
Here, when the toner is not adhered to the photoconductive drum, the toner is brought back to the toner tank and re-collected therein.
For this function, a toner leakage preventing mechanism is provided between the toner tank and the developing roller. The toner leakage preventing mechanism functions as a through pass (from outside to inside) of the collected toner, and also functions to prevent leakage (efflux from inside to outside) of the toner.
FIG. 5 is an explanatory view illustrating a structure of a conventional toner leakage preventing mechanism. As shown in the figure, in this structure, a developing roller 121 is designed to be set in the opening section of the toner tank 124, being rotatable in the H-direction. With this arrangement, the toner leakage preventing mechanism brings the toner having not been adhered to the photoconductive drum (not shown) back into the toner tank 124 through a lower portion (return entrance) of the opening section. Further, in the vicinity of the return entrance, side seals 127 and a front seal board 123 are provided as the toner leakage preventing mechanism.
The side seals 127 are sealing members fixed to the respective sides of the return entrance of the toner tank 124 by double-face adhesive tape or the like. Further, the side seals 127 come in contact with the developing roller 121 with its flexibility, thus preventing leakage of the toner through the end portion of the developing roller 121.
The front seal board 123 is an elastic sheet provided in a lower portion of the return entrance along the longitudinal direction of the developing roller 121. Further, the lower end (lower edge) of the front seal board 123 is fixed to the toner tank 124 while the upper end (upper edge) thereof is a free end. Further, the upper portion is pressed into the developing roller 121, covering the return entrance. With such a front seal board 123, leakage of toner from the lower portion of the return entrance is prevented.
Note that both ends of the front seal board 123 are overlaid on the respective surfaces of the side seals 127.
In regard to such a toner leakage preventing mechanism, Document 1 (Japanese Laid-Open Patent Application Tokukaihei 06-130804/1994 (published on May 13, 1994) discloses a structure in which the surface of the toner tank, that fixes the front seal board, and the surface of the side seal form a continuous plane. This structure is aimed at preventing flexion of the front seal board by the side seal.
In the foregoing mechanism, a stronger force (contact force) of pressing the front seal board and the side seal into the developing roller more securely prevents the leakage of toner.
However, if the force is excessively large, it blocks the through pass for collecting toner, and the running torque of the developing roller needs to be increased. Further, when the developing device is used for a long period of time, such a strong pressing force may induce breakages of the front seal board, the side seal, the toner tank, and/or the developing roller.
Accordingly, the contact force is generally set to a small value that is great enough to seal a gap between the front seal board and the developing roller.
However, with such a small contact force, there is a possibility of generation of a slight gap between the front seal board and the side seal when vibration and/or impact is applied upon shifting of the printing device, or due to secular distortion or the like, thereby causing leakage of toner.
Other possible strategies to prevent such toner leakage may be separation of the toner containing tank from the developing device, or sealing of the opening section of the toner containing tank by a sheet when the printing device is shifted.
However, such separation of the toner containing tank, sealing of the opening section or the like in shifting the printing device required is highly troublesome.
Further, in actual operation, the foregoing methods for preventing leakage of toner may be performed only with respect to a mint (unused) developing device. This is because, once toner is adhered to the developing roller, prevention of leakage of toner cannot be guaranteed.
Further, to adopt the foregoing methods, the toner containing tank needs to be made removable. Otherwise, the opening section needs to be made with a sheet to cover itself as required. Therefore, a complex structure (sealing structure in particular) has to be made between the toner containing tank and the developing roller.